Producing polychloroprene latices

ABSTRACT

The novel process provides latices of homo- and copolymers of chloroprene and of one or more further copolymerizable monomers at a higher reaction rate and hence in an improved space-time yield. The latices obtained have a high gel content, distinctly lower levels of by-products, in particular of chloroprene dimer, than prior art products, and are useful, if appropriate in concentrated form, for a wide variety of applications.

FIELD OF THE INVENTION

The present invention relates to a novel process for producing laticesof homo- or copolymers of chloroprene and if appropriate one or morefurther copolymerizable monomers, to the resulting, similarly novellatices in if appropriate concentrated form, to their use and also tothe products obtained in these uses.

BACKGROUND OF THE INVENTION

Latices of polychloroprene are industrially produced by free-radicalemulsion polymerization of chloroprene (2-chloro-1,3-butadiene), ifappropriate by copolymerization with suitable monomers. Such laticeswill herein also be collectively referred to as “polychloroprenelatices”. The monomers are admixed in an aqueous medium with anemulsifier system in the course of latex production. This emulsifiersystem is generally anionic in nature, but occasionally nonionic orcationic systems are also used. The temperature range in which thepolymerization is carried out comprises values of about 0° C. to about60° C. The polymerization can thus be initiated by thermallydisintegrating free-radical formers, or by means of redox systems.Generally, molecular weight regulators such as mercaptans or xanthogendisulphides are also used. In some cases, the molecular weight of theend product is also set by copolymerizing with sulphur and thensplitting the resulting sulphidic bonds. The desired degree ofconversion is set by stopping the reaction with a suitable agent. In theoverwhelming number of cases, the resulting dispersion ofpolychloroprene in water, i.e. the so-called polychloroprene latex, issubsequently demonomerized by passing steam through it. A portion of theproduct obtained as a result finds direct industrial use as a latex, butthe larger portion is freed of adherent water by coagulation, usuallyfreeze coagulation, and shipped as a solid product to its ultimate use.

The polymerization generally by-produces products such as, for example,dimers of chloroprene or other low molecular weight compounds which canhave an undesirable influence on the odour of the final product, whetherit is to be used as latex or else as a solid rubber. These by-productsare only imperfectly removable by the above-described steam treatmentsin the course of latex devolatilization or else by water wash in thecourse of the work-up of the solid rubber.

One way to reduce the undesirable odorant by-products to some degree atleast consists in conducting the polymerization at a reducedpolymerization temperature. However, a reduced polymerizationtemperature means directly a reduced polymerization rate and thereforeis undesirable for economic reasons. It is further known that thecrystallization tendency of polychloroprene products more oftenmilitates against the planned use.

One technologically important property of synthetic latices is theso-called gel content, which characterizes the fraction of the polymerthat is insoluble in a certain solvent (toluene, for example). A highgel content is indispensable in various applications, for example for ahigh strength of a “wet”, i.e. incompletely dried, film of therespective material. A high gel content is achievable when usingchloroprene (as in the case of all emulsion-polymerized dienes) bycrosslinking by means of multifunctional monomers or by a highconversion for the polymerization reaction. A high conversion (>95% forexample), however, is associated with economically less desirable longpolymerization times, in particular when relatively low polymerizationtemperatures are employed.

Yet an economically better polymerization at relatively hightemperatures, which leads faster to the desired degrees of conversion,results in increased formation of dimeric by-products via Diels-Alderreactions, and there are other secondary reactions observed to takeplace. This frequently gives rise to products which lack utility forsome applications due to the odour nuisance generated during productionor use of the end products.

Various methods have been described in the past of “masking” the odorantby-products by adding scents having a more intense odour. Examples ofsuch masking agents are “scent of hay”, anisaldehyde, vanillin, camphor.This method is described by J. C. Carl in “Neoprene Latex”, E.I. DuPontde Nemours (self-published monograph), 1962, page 67. WO-A-01/00683describes a method comprising the removal of odorants by means of ozoneor hydrogen peroxide. Common to all these methods is that they mask orremove the odorants formed, but do not avoid or minimize theirformation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forproducing polychloroprene latices which proceeds at a high rate ofreaction, yet at the same time avoids the formation of odour-nuisanceby-products and further leads to latices whose technological propertiesare not inferior to those of prior art latices.

We have found that this object is achieved, surprisingly, when theemulsion polymerization is not, as in the prior art, carried out at aconstant temperature setting, but that the polymerization temperature israised in the course of the polymerization.

DETAILED DESCRIPTION OF THE INVENTION

The invention accordingly provides a process for producing a latex ofhomo- or copolymers of chloroprene and if appropriate one or morefurther copolymerizable unsaturated monomers by emulsion polymerization,comprising, proceeding from a starting temperature T_(S), raising thepolymerization temperature during the emulsion polymerization

-   a) by at least 3° C. above the starting temperature T_(S) up to a    conversion of 50% and subsequently-   b) by at least 6° C. above the starting temperature T_(S) up to a    conversion of 90%.

The process according to the invention provides a higher rate ofreaction than the prior art. In addition to this advantage, whichsubstantially enhances the economic attractiveness of the process, itfurther generates a distinctly lower level of by-products than prior artprocesses when the polymerization is carried out at a constant reactiontemperature. Owing to the low level of secondary components, the processis thus also ecologically advantageous.

The process according to the invention comprises an emulsionpolymerization carried out at a pH in the range from 9 to 14 andpreferably in the range from 11 to 13. The pH is set with customaryadditives, preference being given to KOH or NaOH.

The polymerization is initiated using conventional initiators orinitiator systems. Examples of initiators and initiator systems are:formamidinesulphinic acid, potassium peroxodisulphate, redox systemsbased on potassium peroxodisulphate and if appropriate silver salt(sodium salt of anthraquinone-beta-sulphonic acid), examples of redoxpartners being compounds such as formamidinesulphinic acid, the sodiumsalt of hydroxymethanesulphinic acid, sodium sulphite and sodiumdithionite. Redox systems based on peroxides and hydroperoxides are alsosuitable. The customary amounts to be used of the initiator/initiatorsystem can easily be determined by a person skilled in the art on thebasis of his or her expertise, depending on factors such as, forexample, the size of the reaction vessel or of the tank or reactor.

In accordance with an essential feature of the invention, thepolymerization temperature is raised in the course of the emulsionpolymerization reaction, from a starting temperature T_(S).

The starting temperature T_(S) and the polymerization temperaturetypically range from 0° C. to 70° C., preferably from 10° C. to 60° C.and more preferably from 35° C. to 50° C.

In the process according to the invention, proceeding from a startingtemperature T_(S), it is important that the polymerization temperatureis raised

-   a) by at least 3° C. above the starting temperature T_(S) up to a    conversion of 50% and subsequently-   b) by at least 6° C. above the starting temperature T_(S) up to a    conversion of 90%.

In a preferred embodiment, proceeding from a starting temperature T_(S),the polymerization temperature is raised

-   a) by at least 5° C. above the starting temperature T_(S) up to a    conversion of 50% and subsequently-   b) by at least 6° C., preferably by at least 8° C., above the    starting temperature T_(S) up to a conversion of 90%.

In other words:

If the starting temperature T_(S) is increased by at least 3° C. up to aconversion of 50%, the temperature is thereafter increased once more byat least 3° C. up to a conversion of 90%.

If the starting temperature T_(S) is preferably increased by at least 5°C. up to a conversion of 50%, the temperature reached at that point issubsequently increased once more by at least 1° C. and preferably by atleast 3° C. up to a conversion of 90%.

The raising of the polymerization temperature in the process accordingto the invention can be effected either in stages or alternativelycontinuously.

With this specific form of temperature management in the processaccording to the invention, the temperature at the start is usuallylower than that of existing, isothermal emulsion polymerizations forproducing polychloroprene latices and is then gradually raised only forthe temperature at the end of the polymerization reaction being elevatedcompared with the prior art processes. It will be found advantageous toengineer the temperature increases in the process according to theinvention such that the average polymerization temperature resultingover the polymerization time comes close or is equal to the isothermaltemperature of a prior art reaction management.

The process according to the invention provides latices based onpolychloroprene or based on copolymers of chloroprene and, ifappropriate, one or more further copolymerizable unsaturated monomers.

Useful copolymerizable unsaturated monomers for the production ofcopolymers are described in “Houben-Weyl: Methoden der OrganischenChemie”, Volume 20, Part 3, Macromolecular Materials, 1987, page 844.Preference is given to monomers having 3 to 12 carbon atoms and 1 or 2copolymerizable C═C double bonds per molecule. Examples of preferredcopolymerizable monomers are 2,3-dichlorobutadiene and1-chlorobutadiene.

The process according to the invention can be carried out eitherbatchwise or else continuously. Continuous polymerization is preferred.Continuous operation can also be realized for example by seriesconnection of various reaction vessels or tanks with increasingpolymerization temperature.

The way in which emulsion polymerization is carried out in principle isknown to a person skilled in the art. Customarily, the aqueous phasecontaining the emulsifier system and the monomer or monomers areintroduced into a reaction vessel as initial charge and are subsequentlypurged with an inert gas, preferably nitrogen. The reaction system isthen brought or heated to the desired starting temperature for thepolymerization and the polymerization is started by addition of theinitiator or initiator system. The addition of the initiator orinitiator system can be done in one or more portions, or elsecontinuously, depending on type. After the desired degree of conversionhas been reached, the polymerization is customarily stopped by additionof a suitable agent. Useful stoppers include in particulardiethylhydroxylamine or phenothiazine.

Preferably, after the polymerization, the residual chloroprene and alsothe further optional monomers used are removed. This is done by steamdistillation for example. A steam distillation is carried out forexample as described in “Houben-Weyl: Methoden der organischen Chemie”Volume 20, Part 3 Macromolecular Materials, (1987) pages 842-859.

The aqueous polychloroprene homo- or copolymer dispersions (theso-called latices) obtained by the process according to the inventionare likewise novel.

The present invention therefore further provides an aqueous dispersion(latex) of a homo- or copolymer of chloroprene and if appropriate one ormore further copolymerizable unsaturated monomers, obtainable by theprocess according to the invention, this homo- or copolymer having a gelcontent in toluene at 23° C. of greater than 60% by weight, based on thehomo- or copolymer, preferably of greater than 80% by weight and morepreferably of greater than 90% by weight.

The so-called gel content is a parameter familiar to a person skilled inthe art. The gel content is that fraction of polychloroprene latex thatis insoluble in toluene at 23° C. To determine it, 250 mg of thepolychloroprene latex are swollen in 20 ml of toluene by shaking at 23°C. for 24 hours. After centrifugation at 20 000 revolutions per minute,the insoluble fraction is separated off and dried. The gel content iscalculated as the ratio of the dried residue to the initial weight andis reported in per cent by weight.

When a copolymer of chloroprene and one or more further copolymerizableunsaturated monomers is produced in the process according to theinvention, this copolymer preferably has a chloroprene content of atleast 20% by weight, preferably 50% by weight.

The aqueous dispersion (latex) according to the invention thus isnotable for the aforementioned high gel content of the polymer and alsofor a very low fraction of undesirable secondary components, inparticular chloroprene dimers. These dimers of chloroprene comprise 6structurally isomeric compounds. The latices therefore have only a verylow odour and, what is more, are obtainable by an economicallyadvantageous process.

Odour comparisons between various substances, under identical definedconditions, can be carried out by sensory testing. In the present caseof the polychloroprene latex according to the invention, it is furtherpossible to carry out an assay of chloroprene dimers by gaschromatography, these dimers serving as index substance for the odour.

The polychloroprene latices according to the invention can be furtherconcentrated. Customarily, for this purpose, the aqueous dispersions ofthe homo- or copolymers of chloroprene and if appropriate one or morefurther copolymerizable unsaturated monomers are concentrated bycoagulation. Such methods of coagulation are well known to a personskilled in the art. Freeze coagulation is preferred, removing adherentwater and obtaining a concentrated latex in the form of a solid product.

The invention thus also provides a concentrated latex of a homo- orcopolymer of chloropropene and if appropriate one or more furthercopolymerizable unsaturated monomers, the homo- or copolymer having agel content in toluene at 23° C. of greater than 60% by weight, based onthe homo- or copolymer, preferably of greater than 80% by weight andmore preferably of greater than 90% by weight.

The polychloroprene latices according to the invention and similarly theconcentrated polychloroprene latices are very useful for variousapplications.

The invention accordingly further provides for the use of theinvention's polychloroprene latices, if appropriate in concentratedform, for producing waterborne disperse adhesives, the polychloroprenelatices, if appropriate in concentrated form, being used either as solelatex component or in admixture with other latices.

It is very advantageous for this use that latices are notable for lowintrinsic odour while at the same time possessing high tack and highthermal stability.

In this use, the polychloroprene latex, if appropriate in concentratedform, is combined with water and optionally further additives to producean aqueous dispersion of an adhesive. The disperse adhesive sets whenthe water evaporates and the particles of polymer flow together and forman uninterrupted film.

The present invention further provides for the use of the invention'spolychloroprene latices, if appropriate in concentrated form, forproducing moulded foam articles, for bonding all types of fibres, forimpregnating fabrics, for enhancing bitumen or bituminous materials, forproducing dipped goods, the polychloroprene latices (if appropriate inconcentrated form) being used either as sole latex component or inadmixture with other latices.

When the polychloroprene latex (if appropriate in concentrated form) isused for producing moulded foam articles, the polychloroprene latex issubjected to a foaming operation.

When the polychloroprene latex (if appropriate in concentrated form) isused for impregnating fabrics, the polychloroprene latex is applied to afabric.

When the polychloroprene latex (if appropriate in concentrated form) isused for bonding fibres, fibres of natural origin or synthetic natureare treated with the polychloroprene latex.

When the polychloroprene latex (if appropriate in concentrated form) isused for enhancing bitumen or bituminous materials, bitumen orbituminous materials are treated with the polychloroprene latex.Enhancing here is to be understood as referring to the elasticizing ormodifying of the bitumen or of the bituminous materials.

When the polychloroprene latex (if appropriate in concentrated form) isused for producing dipped goods, a dipping form is introduced into acoating bath containing the polychloroprene latex, a surface film formson the dipping form, the dipping form is subsequently removed from thecoating bath and the surface film which has formed on the dipping formas a result of being wetted with the latex according to the invention issubjected to a vulcanization.

Again, it is of great advantage for all the aforementioned uses that thelatices are notable for low intrinsic odour.

When the invention's polychloroprene latices (if appropriate inconcentrated form) are used in the aforementioned applications inadmixture with other latices, all customary and known other latices areuseful as further components. For example, SBR, acrylate or polyvinylacetate latices are useful as further components.

The production of disperse adhesives based on polychloroprene latex isdescribed for example in “Handbook of Adhesives”, published by vanNostrand Reinhold, Third Edition, New York 1990, pages 301-305. Themonograph by J. C. Carl (loc.cit.) gives a summarizing account of thetypical latex applications such as moulded foam production, fabricimpregnation, fibre bonding, dipped goods production and bitumenenhancement.

The present invention further provides

-   -   the disperse adhesives based on the polychloroprene latices        according to the invention,    -   the moulded foam articles based on the polychloroprene latices        according to the invention,    -   the fabrics impregnated with the polychloroprene latices        according to the invention,    -   the fibres bonded with the polychloroprene latices according to        the invention,    -   the bitumen or bituminous materials enhanced with the        polychloroprene latices according to the invention, and also    -   the dipped goods based on the polychloroprene latices according        to the invention.

EXAMPLES Example 1 (Comparative) and Example 2 (Inventive) a) Recipesfor Examples 1 and 2

The emulsion polymers were produced using the following stock recipe(quantities reported in parts by weight per 100 parts by weight ofchloroprene used):

Example 1 (comparative) Example 2 Chloroprene 100 100 Constituents ofaqueous phase: Ion-free water 100 100 Disproportionated resin acid,sodium salt 3.5 3.5 (dehydro-, dihydroabietic acid Na salt) Condensationproduct of 1.0 1.0 naphthalenesulphonic acid and formaldehyde (Na salt)sodium hydroxide Activator (formamidinesulphinic acid) 0.36 0.38

b) Procedure of Emulsion Polymerization Example 1 Isothermal Operation(Comparative)

A 4 l reactor was charged with the aqueous phase (1257 g) and themonomer phase (1200 g) as initial charge and this initial charge waspurged with nitrogen. After the emulsion had been heated to 44° C., thepolymerization was started by addition of a small amount (20 ml) of 2%aqueous formamidinesulphinic acid and continued by continuous furtheraddition of the same solution to a conversion of 95%. After this degreeof conversion was reached, the polymerization was stopped by addition of0.1 part of diethylhydroxylamine in the form of an aqueous solution(10%). Unconverted monomers were removed under reduced pressure by meansof steam down to a residual monomer content of about 800 ppm. Theconversions reported in Table 1 were gravimetrically determined onsamples taken from the polymerization reactor at the respective timesreported in Table 1.

Example 2 Inventive

Example 1 was repeated except that the starting temperature for thepolymerization was chosen to be 40° C. and was gradually raised duringthe polymerization as reported below in Table 2. The conversionsreported in Table 2 were gravimetrically determined on samples takenfrom the polymerization reactor at the respective times reported inTable 2.

c) Experimental Results

Tables 1 and 2 show the results of the conversion determinations in thecourse of the polymerizations of Examples 1 and 2 and also therespective polymerization temperatures.

TABLE 1 Example 1 (comparative) Example 2 (inventive) PolymerizationMonomer Polymerization Monomer Polymerization time conversiontemperature conversion temperature (h) (%) (° C.) (%) (° C.) 0  0 44 040 1.0 40 44 30 40 1.5 65 44 55 42 2.0 73 44 75 44 3.0 82 44 85 46 3.25— — 95 48 4.0 89 44 5.5 95 44

Comparing Inventive Example 2 and Comparative Example 1 immediatelyreveals the advantage of the invention that there is a distinct increasein the space-time yield to be observed.

d) Gel Content

The gel content of the polymers of the two examples was determined bydetermining that fraction of the polychloroprene latex that is insolublein toluene at 23° C. To determine it, 250 mg of the polychloroprenelatex are swollen in 20 ml of toluene by shaking at 23° C. for 24 hours.After centrifugation at 20 000 revolutions per minute, the insolublefraction is separated off and dried. The gel content is calculated asthe ratio of the dried residue to the initial weight and is reported inper cent by weight.

The result recited in Table 2 shows that the process according to theinvention gives similar gel contents, suggesting a similar polymericstructure.

TABLE 2 Example Gel content in toluene (% by weight) 1 (comparative) 852 (inventive) 84

e) Sensory Testing

Of the latices obtained in Examples 1 and 2, 500 ml at a time wereintroduced into a 11 wide neck flask, which was sealed and stored atroom temperature for 24 h. After opening, the respective sample wasodour-sensorially tested by three judges. They rated their perception ofthe odour against the following scale:

-   -   1=very good    -   2=good    -   3=neutral    -   4=unpleasant    -   5=very unpleasant

The judges' ratings are summarized in Table 3.

TABLE 3 Example Judge A Judge B Judge C 1 (comparative) 3 4 4 2(inventive) 2 2 3

The judges' assessment was consistently more positive for the inventivelatex.

f) Dimer Contents

The latices of Example 1 and Example 2 were subjected to agas-chromatography test for dimer content (dimers of chloroprenemonomer). The by-produced dimers comprise 6 structurally isomericcompounds, which are grouped together. The average dimer contentdetermined from 5 gas-chromatographic assays per example is reported inTable 4:

TABLE 4 Example Dimer content 1 (comparative) 487 ppm 2 (inventive) 254ppm

Again, the distinctly reduced content of dimers—used as index substancefor an ecological evaluation—demonstrates the advantage of the inventivepolychloroprene latices.

1. A latex of a homo- or copolymer of chloroprene and if appropriate oneor more further copolymerizable unsaturated monomers, prepared byproceeding from a starting temperature T_(s) raising the polymerizationtemperature during the emulsion polymerization a) by at least 3° C.above the starting temperature T_(s) up to a conversion of 50% andsubsequently b) by at least 6° C. above the starting temperature T_(s)up to a conversion of 90%, wherein the homo- or copolymer has a gelcontent in toluene at 23° C. of greater than 60% by weight, based on thehomo- or copolymer and wherein the latex has a chloroprene dimer contentof less than 254 ppm.
 2. The latex of a homo- or copolymer ofchloroprene and if appropriate one or more further copolymerizableunsaturated monomers, according to claim 1, wherein, the homo- orcopolymer having a gel content in toluene at 23° C. of greater than 80%by weight, based on the homo- or copolymer.
 3. A concentrated latex of ahomo- or copolymer according to claim 1 or
 2. 4. A method of producing awaterborne disperse adhesive comprising mixing the polychloroprene latexaccording to claim 1 either as sole latex component or in admixture withother lattices with water and evaporating the water.
 5. A method ofproducing molded foam articles comprising subjecting the polychloroprenelatex according to claim 1 to foaming either as sole latex component orin admixture with other latices in a mold.
 6. A method of producingbonded fibers, impregnated fabrics, enhanced bitumen or bituminousmaterials or dipped goods, comprising contacting the polychloroprenelatex according to claim 1 either as sole latex component or inadmixture with other lattices with fibers, fabrics, bitumen oderbituminous materials or goods.
 7. Disperse adhesive based on apolychloroprene latex according to claim
 1. 8. Molded foam article basedon a polychloroprene latex according claim
 1. 9. Fabrics impregnatedwith a polychloroprene latex according to claim
 1. 10. Fibers bondedwith polychloroprene lattices according to claim
 1. 11. Bitumen orbituminous materials enhanced with a polychioroprene latex according toclaim
 1. 12. Dipped goods based on a polychioroprene latex according toclaim
 1. 13. A latex of a homo- or copolymer of chloroprene according toclaim 1 wherein the chloroprene dimers comprise 6 structurally isomericcompounds.